Thermosetting topcoat compositions comprising a hydroxyl-containing acrylic resin and a melamine resin have been heretofore chiefly used for coating automotive exterior panels. However, in recent years, acid rain has posed a world-wide problem of etching and blots or like stains on coating films. With the widespread use of car washers, scratches made on coating films by car washers have presented another problem. In this situation, there is a need for topcoat compositions, particularly clear coat compositions, which are capable of forming coating films satisfactory in both acid resistance and scratch resistance.
Usually, the scratch resistance of a coating film can be improved by increasing the crosslinking density of the film. On the other hand, the acid resistance of a coating film can be improved by incorporating an acid resistant crosslinking system into the film. However, a method has been scarcely proposed for giving both acid resistance and scratch resistance to a coating film.
For example, Japanese Unexamined Patent Publication No. 222,753/1990 discloses that a coating film having high crosslinking density, and thus having high scratch resistance, can be obtained by baking an acrylic resin of a high hydroxyl value in the presence of a monomeric melamine resin and an acid catalyst. However, this crosslinking system has a drawback of poor acid resistance since the coating film crosslinked by a melamine resin readily decomposes when contacted with an acid.
Methods have been proposed for imparting both acid resistance and scratch resistance to a coating film by incorporating an acid-resistant crosslinking system to a less acid-resistant melamine resin crosslinking system. The proposed systems include, for example, a composite crosslinking system having a combination of carboxyl group/epoxy group/hydroxyl group/melamine resin (Japanese Unexamined Patent Publication No. 247,264/1990), a composite crosslinking system having a combination of hydroxyl group/alkoxysilyl group/melamine resin (WO91/16,383), etc. However, these systems do not always achieve satisfactory improvements in acid resistance because of the presence of a melamine resin.
On the other hand, melamine resin-free crosslinking systems have been proposed. The proposals include a crosslinking system having only a combination of carboxyl group/epoxy group or carboxyl group/epoxy group/hydroxyl group (e.g., Japanese Unexamined Patent Publications Nos. 87,288/1987, 45,577/1990 and 287,650/1991). Yet, these systems have a shortcoming that the resulting coating films, although superior in acid resistance, are inferior in scratch resistance owing to the low crosslinking density of the cured film.
A resin composition comprising a carboxyl-containing acrylic polymer and an epoxy- and hydrolyzable silyl-containing compound is known as a crosslinking system having a combination of carboxyl group/epoxy group/hydrolyzable silyl group (Japanese Unexamined Patent Publication No. 187,749/1987), but this composition has a defect that the curing reaction of the polymer with the compound is unsatisfactory because of the steric hindrance caused by the presence of the epoxy group and the hydrolyzable silyl group in the same molecule.
A resin composition comprising a hydroxyl- and carboxyl-containing silicone polymer, a carboxyl- and carboxylic acid ester group-containing polymer and a hydroxyl- and epoxy-containing polymer has been proposed as a crosslinking system having a combination of carboxyl group/epoxy group/hydroxyl group which contains the silicone polymer as a base resin (Japanese Unexamined Patent Publication No. 166,741/1994). However, the cured coating of the composition is defective in that it is not always fully satisfactory in crosslinking density and Is poor in the recoat adhesion that is one of the important characteristics of coating compositions for automotive exterior panels.
On the other hand, it is of urgent necessity in the field of coating compositions to take measures for the control on the use of organic solvents, from the viewpoints of prevention of air pollution and conservation of resources. Such measures include development of high solids coating compositions that contain a less amount of solvents and have a higher solids concentration.